fs: don't conflate data and callback in appendFile

PR-URL: nodejs#11607
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/fs.js

@@ -1320,7 +1320,7 @@ fs.writeFileSync = function(path, data, options) {
 };
 
 fs.appendFile = function(path, data, options, callback) {
-  callback = maybeCallback(arguments[arguments.length - 1]);
+  callback = maybeCallback(callback || options);
   options = getOptions(options, { encoding: 'utf8', mode: 0o666, flag: 'a' });
 
   // Don't make changes directly on options object

test/parallel/test-fs-append-file.js

@@ -144,6 +144,13 @@ fs.open(filename5, 'a+', function(e, fd) {
   });
 });
 
+// test that a missing callback emits a warning, even if the last argument is a
+// function.
+const filename6 = join(common.tmpDir, 'append6.txt');
+const warn = 'Calling an asynchronous function without callback is deprecated.';
+common.expectWarning('DeprecationWarning', warn);
+fs.appendFile(filename6, console.log);
+
 process.on('exit', function() {
   assert.strictEqual(12, ncallbacks);